The present invention relates to wireless data communication networks.
Wireless data communication networks are widely used. For example, popular types of wireless local area network (WLAN) are based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard or its variations. Typical applications of 802.11 WLANs include office, enterprise and home data networks.
The 802.11 media access control (MAC) layer protocol uses a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) approach to provide random access to the network for all devices while reducing contention that results from overlapping transmissions by more than one device at a time. In CSMA/CA, a device that wishes to transmit a frame listens to the carrier frequency for a fixed interval to make sure the channel is idle, and then starts transmission. When the receiving device receives the frame, the receiving device sends back an acknowledgement frame to the sending device by following the same listen-before-transmission procedure. In addition, in some cases a round of messages consisting of RTS (request to send) and CTS (clear to send) may take place before data is actually exchanged.
The overheads occasioned by the 802.11 protocol are generally not large when the amount of data communicated in each data frame is substantial, as is usually the case in enterprise networks.
It has also been proposed to employ WLANs in industrial control applications to replace wired networks and connect field devices to industrial system controllers. Typically the number of field devices may be large, e.g., in the hundreds, and the typical size of each message may be small, say a frame size of 64 bytes. If the 802.11 protocol were applied without change to such applications, the overhead may be quite high, e.g., in excess of 80%, and it may not be possible to support the desired number of field devices.